Magnetic media drives, such as disk drives, have heads mounted on actuator arms that are cushioned on an air bearing surface during normal operation. When a drive's head actuator assembly is in a read/write position it does not make contact with the media surface because of the air bearing. When boundary condition events occur, such as, a power down, a sudden power loss, a spindown, or a system generated retract command, the heads must be moved to a landing zone to protect data integrity because air bearing loss is imminent. Any contact between head and media over the data zone has the potential of damaging the media surface, the head, or causing localized media demagnetization due to impact forces.
Also head and disk contact could generate debris within the head-disk assembly (HDA) reducing head and disk interface reliability, eventually causing head crashes and data loss. The most stringent boundary conditions, such as power loss, result in spindown where normal drive power becomes unavailable to a disk drive. When that happens, the rectified back emf of the spindle motor must supply the necessary power to move the heads to a "landing" zone on the media's surface suitable for head and disk contact. This function of moving heads to a safe zone is generally referred to as "retract" or "park".
Further, the spindle generated back emf is proportional to spindle speed and upon loss of power the spindle speed decreases rapidly causing the spindle generated back emf to drop rapidly as well. Hence the spindle has only finite amount of stored energy. A retract circuit has an operating voltage range and hence the actuator must complete retract before the spindle generated back emf voltage drops below the range. Therefore, there exists a time limit on the retract duration because the entire retract operation has to be completed before the operating range drop-out voltage is reached.
In the case of a magnetic disk, the landing zone is a highly polished area of a disk where no data is stored. The landing zone is provided so that a head can be parked there, i.e., the head actuator assembly can actually make contact with the disk surface without causing damage either to the data stored on the magnetic media, or to the media, or to the head itself.
Further, two crash stops, an inner crash stop and an outer crash stop are also provided to prevent head actuator assemblies from flying off the disk surface if the disk drive actuator/servo electronics loses control. One of these crash stops is used to locate the landing zone. During high velocity seeks, especially near the crash stops, a boundary condition event will cause loss of normal control. Unless attenuated, the high actuator velocity will cause a high force impulse contact with a crash stop. This sudden deceleration, could cause heads to twist on their flexure arms, overcome the air bearing, and subsequently make debris while generating disk contact. Hence there is a need for a solution that minimizes potential data loss, and head/media damage resulting from a head actuator assembly impacting crash stops at high velocities for boundary condition events.
Three prior art methods are known for controlling impact velocity while moving the head actuator assembly to a landing zone. One such method uses a unipolar fixed voltage, sourced from single quadrant circuitry capable of only sourcing current. This unipolar fixed voltage is applied to an actuator's coil to move the actuator assembly over to the landing zone. However, this method has its drawbacks. During a high velocity seek, when the actuator is moving at a high velocity towards the landing zone, an actuator's velocity cannot be reduced by this fixed voltage technique because the circuitry cannot sink current. Also, when the actuator is moving at high velocity away from the landing zone, velocity attenuation capability is severely limited by the circuit's high source resistance. Therefore, this fixed voltage method doesn't provide adequate high velocity attenuation and subsequent impact protection.
The second method, known as dynamic braking, also uses a unipolar fixed voltage, but it is supplied by two quadrant circuitry capable of both sourcing and sinking current. Again a unipolar fixed voltage is applied to an actuator's coil to move the actuator assembly over to the landing zone. During a high velocity seek, moving in either direction, the two quadrant circuit effectively provides a very low impedance path between the coil terminals. The coil back emf generates a current through this path that attenuates actuator velocity. However, the prime drawback is that attenuation is limited by the actuator's back emf voltage and circuit resistances. Therefore, although the actuator is dynamically braked, it is not enough to provide adequate impact protection. This is especially apparent when a high velocity seek occurs near a crash stop. Further, using this method requires longer crash stop zones to allow more deceleration distance, thereby reducing disk data storage capacity.
The third prior art method uses a back-emf feedback velocity control loop to regulate the impact velocity. This method requires a closed loop control system which is unnecessarily complex and poses some risk. The risk involves matching loop compensation parameters with high tolerance, high temperature variant circuit components. This presents a closed loop stability problem. Attempts to improve stability margins by detuning the control loop results in poor velocity control and subsequent poor retract performance.
Hence, there is a need for a mechanism for quick retraction of an actuator from a read/write position to a landing zone, with significant reduction in crash stop impact velocities, and in a controlled manner, during power down, or power loss, or spindown, or a system retract command. Further, there is a need for a retraction mechanism that overcomes prior art problems of control loop stability, poor actuator velocity control, poor dynamic braking capabilities and inadequate high velocity impact protection.